Subsea immersed tunnel construction is to place multiple immersed tube sections on an underwater gravel foundation bed surface one by one. Accumulation of seabed mud on the gravel foundation bed surface will change the stress characteristics of the gravel foundation bed surface and affect the force transfer effect of a foundation bed structure, thus leading to a phenomenon that the immersed tube sections may not be in effective contact with the gravel foundation bed surface; and during the placement of the immersed tube sections, it needs to dredge placement positions on the gravel foundation bed surface to prevent the phenomenon that the immersed tube sections may not be jointed with adjacent tube sections due to their abnormal settlement when placed on the mud.
In order to efficiently pave the underwater gravel foundation bed surface, an existing mechanical workboat is used for riprapping and pavement; a riprapping tube extends into water and is used as a surface region; the riprapping tube dumps rocks while moving, thus paving a Z-shaped gravel ridge foundation bed surface; a furrow is reserved between two adjacent gravel ridges; and a conventional dredging system may hardly efficiently clear away the mud in the furrows.